Conventional ultrasound imaging systems comprise an array of ultrasonic transducer elements which are used to transmit an ultrasound beam and then receive the reflected beam from the object being studied. For ultrasound imaging, a one-dimensional array typically has a multiplicity of transducer elements arranged in a line and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducer elements can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam. Multiple firings may be used to acquire data representing the same anatomical information. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. By changing the time delay and amplitude of the applied voltages, the beam with its focal point can be moved in a plane to scan the object.
The same principles apply when the transducer array is employed to receive the reflected sound (receiver mode). The voltages produced at the receiving transducers are delayed and summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting a separate time delay (and/or phase shift) and gain to the signal from each receiving transducer.
A phased-array ultrasound transducer consists of an array of small piezoelectric elements, with an independent electrical connection to each element. In most conventional transducers the elements are arranged in a single row, spaced at a fine pitch (one-half to one acoustic wavelength on center). As used herein, the term "1D" array refers to a single-row transducer array having an elevation aperture which is fixed and a focus which is at a fixed range; the term "1.5D" array refers to a multi-row array having an elevation aperture, shading, and focusing which are dynamically variable, but symmetric about the centerline of the array; and the term "2D" array refers to a multi-row transducer array having an elevation geometry and performance which are comparable to azimuth, with full electronic apodization, focusing and steering. Electronic circuitry connected to the elements uses time delays and perhaps phase rotations to control the transmitted and received signals and form ultrasound beams which are steered and focused throughout the imaging plane. For some ultrasound systems and probes, the number of transducer elements in the probe exceeds the number of channels of beamformer electronics in the system. In these cases an electronic multiplexer is used to dynamically connect the available channels to different (typically contiguous) subsets of the transducer elements during different portions of the image formation process.
With the advent of multidimensional probes and the high element counts required to create this type of diagnostic imaging device, the problems associated with the manufacture of ultrasonic transducer arrays are mounting. The problem of "dead" (i.e., fully inoperative) or "weak" (i.e., partially inoperative) elements is of particular concern as the industry moves toward the manufacture of two-dimensional arrays having element counts exceeding 1,000. Process or material errors are often responsible for the creation of a probe without all elements fully functional. Because the specifications regarding element functionality are quite tight, there is little tolerance for dead or weak elements in current probe manufacturing. These requirements focus particular attention on the performance of the manufacturing processes and directly control the amount of manufacturing scrap. Dead element specifications are tight because the effect of this defect on the image quality can be significant, particularly in the near field of the image where a fewer number of elements are used to form the beam. In this case a dead or weak element becomes a significant part of the aperture used to create the image, resulting in an intensity loss at the acoustic line in question.
Currently, probe technology is delivering element counts on the order of 200 or less with processes providing reasonable manufacturing yields. However, new technology is demanding probes with element counts greater the 1,000. Utilizing current production processes, the dead or weak element probability delivers unacceptable yield at this element density. Other than developing new or tighter tolerance manufacturing processes, there are two solutions to this problem. The first is to develop a compensation scheme within the system to detect the presence of the dead element and adjust the beamforming parameters to compensate for the loss. One such technique is disclosed in U.S. Pat. No. 5,676,149 to Yao. The second is to detect the failure during or after the manufacturing process and connect the missing element with a functional one, thereby reducing or eliminating the effect of the failure.